Antenna



.flecting surface, of small axial Patented May 20, 1952 William sank, Lynahurefi. in, mesne assignments, to

.enfo r. lit n ece-area States of America as represented by the Secretary of War Application Aprii'ao, 1943, serial 'N'o. sens came. (ct'z'd s'sittl This, inventionrelates to antennas for high frequency communication systems including radio object-locating radar) systems and more particularly to a means for illuminating a beamformingor beam-directing reflector. V H

In certain radarlsystems, for example, ground search. apparatus or airborne search systems, it is desirablevthat the antenna produce a penciltype beam which is characteristically narrow in one or both of the azimuthal or elevational planes. This hasbeen accomplished by feeding the radiantenergy into a pill-box type reflector which inturn feeds energy to and illuminates a secondary reflector. A pill-box reflecting element generally comprisesa parabolic cylindrical redimension, mounted vbetweenhplane ,parallel metallic walls orplates whichserve to enclose the ends of the parabolic cylinder. ,The radiant energy is fed into the'pill-box by means of an enclosed transmissionline such as a waveguide, if desired with .a horniypetermination, opening into the pillbox. .Heretoforathe end of the transmission line orthe horn termination has been located at the .approximatefocalpoint of the parabolic reflecting surface and within the pill-box itself. A

'pill-box'reflecting surface is particularly suitable .forilluminating a parabolic cylindrical reflector as it gathers the radiant energy into a primary beam of 'subs'tantially;uniform width in the horiz'ontal'plane, .thatis, a beam which isrelatively wide. in, the horizontal plane and narrow in the ve'rticalplane (when thepill-box is oriented with theaxis ofthe pill-box cylinder vertical) This energyis directediontothe secondary reflector whichmay be of, any desired configuration for producing any desired radiation pattern including one having a csc 0 configuration. While an antennahaving such an arrangement has proved .generally satisfactory, considerable difiiculty has been experienced. with, pressurizingv the feed withindthegpill-box.and in matching the impedances. so as to, prevent unwanted reflections of energy from .the pill-box back into the feedline.

It isfone. of the objects of the present inventionto'provide an antenna having a pill-box type primary reflector for directing radiant energy onto a secondary reflector in which theantenna pill box .which in turn reflects energy in, a flat beam pattern onto a secondary reflector shaped componentsiare so arranged that the difiiculties to produce a radiation psaan sw confignra tion in which the feed and pillf-boxfarejso located as to facilitate pressurizin'g and impedance t hingfof the ree 61 a 'better iunderstanding for the invention present inventiomand ig. is a side sectional view of the same. Fig

'Zaisaside sectional view showing a modification of one elementof the present invention. Fig 2b. isfa lsid 'eg sectional View s showing a modification of another element of the present invention. H t h V .JnQthe drawing, the alnte'i'ina I0 is shown in simplifiedform and comprises a feeding translmi's sionline, which may be in the form of a rectangular-waveguide ILYa pill-box type primary i' eflectorfll and a secondary or beam-shaping re- ,flctorlt. 'In'tlhep'osition as shown, the antenna .IBfis adapted jto ibejsupported from the under (serrat of an aircraft and to direct the radiatiolnpattern'a'head (to the right) of secondaryr reflector |'3. and.ldownwardly towards the earth s surface, It [will be unde'rstood, however, lthajjt while the present description is particularly directed to the antenna in the position indicated for airborne it m ay be reversed in position ri s qfl gi q ipmen h ill-b x e. fmar r fl t 1 .5m prises a raboiic cylindrical reflecting surface "l4 of man faXial dirnerision, the. axis being oriented vertically Preferably the planespf the opposite ends of parabolic cylinder 14 are parplane surface if desired bottom plate l5 may ,berpm tted. h t. the p rab c cyl d r is c o ed at u ensndby tq 'plate [6 on y. w thout appreciably 'afiecting the {primary radiation "pattern, Energy is fed from a source (not shown) to pill-box I2 by means of an enclosed transmission line which as shown is preferably in the form of a waveguide II having a rectangular shape in cross-section. Waveguide II extends through a suitable aperture I8 in top plate I6 and is then bent toward pill-box I2 so that the plane of its mouth I! at the free end thereof is substantially perpendicular to the top plate I6 and spaced from and in parallel relation to the plane of aperture I I. Thus, the mouth of waveguide I I, which may be in the form of a flared horn having mouth IQa, shown in Fig. 2a, is located outside of pillbox I2, instead of entering the aperture I'I thereof in the manner heretofore utilized, while being at the approximate focal point of parabolic cylindrical surface I4. Inasmuch as the metallic top plate I6 itself acts as part of mouth I9 or horn mouth I 9a and is thus effective to shape the primary pattern of energy issuing from mouth I9 or horn mouth I911 onto the parabolic surface I4, mouth I9 or horn mouth I9a may be of relatively small dimensions.

Secondary reflector I3 may be shaped to produce any desired beam pattern by reflecting energy directed thereon by pill-box I2. It is often desirable that the energy distribution in a vertical plane be such that the variation of energy density versus the radiation angle approximates a cosecant-squared function. This distribution provides generally uniform echo strength for targets disposed at substantially equal elevations with respect to the antenna regardless of whether the target is close in or distant. Thus, secondary reflector I3 preferably comprises a portion of a generally cylindrical reflector having its reflecting surface I3 shaped according to geometric optics to produce a radiation pattern closely approximating the relation csc in elevation, where 0 is the radiation angle measured from the axis of an undistorted or symmetrical beam pattern, i. e., a pattern normally produced by an undistorted or conventionally shaped reflector. Reflecting surface I3 may therefore comprise a portion formed as a parabolic surface for directing the energy beam along a main path with another portion (as shown the upper portion) having a modified nonparabolic shape adapted to direct a component of the energy along a divergent path whereby an unsymmetrical radiation pattern which generally varies as the square of the cosecant of the angle, measured from the axis of the radiation pattern emanating from pill-box I2, is produced.

Reflector I3 is preferably located so that its focal line, i. e., the locus of its focal points, normally lies in a horizontal plane preferably also lying substantially in the plane of the aperture I! of pill-box I2. Thus, the energy reflected from parabolic surface I4, the pattern of which is generally wide in azimuth and narrow in elevation to give a substantially flat beam pattern substantially parallel to plate I6, is adapted to illuminate and be reflected from reflecting surface I3 in a radiation pattern which is substantially uniform in azimuth and which is of csc 0 configuration in elevation.

The top plate I6 preferably extends from pillbox I2 to reflector I3 and may actually be attached along the top edge 20 of reflector I3. Alternatively, the reflector I3 may approach close to the top edge 20 and be spaced slightly therefrom to permit reflector I3 to move relative thereto. For example, reflector I3 may be rotatable or tiltable about a horizontal axis, as shown in Fig. 2b, whereby the distribution pattern may be varied to obtain substantially uniform gain and echo strength in the ground plane for different altitudes of the antenna when the latter is airborne. For satisfactory operation, the depth of reflector I3 that is the distance between the longitudinal edges thereof should be approximately twice the axial length of the pill-box I2.

With the antenna arrangement as hereinbefore described the plane of polarization of the radiant energy as it issues from the mouth I9 or horn mouth IHa of waveguide II is parallel to the top plate I6, as indicated by the arrows in Fig. 1, and there is a substantially zero electrical field along the metallic lower surface of plate I5. Consequently, very little power will be reflected back into mouth I9 or horn mouth I9a. Thus, with the present arrangement, it is very easy to match the horn to space and to the impedance and pillbox I2. Furthermore, as the waveguide feed or horn radiating element II is disposed outside of the pill-box I2, it maybe easily pressurized according to well-known methods, such as by sealing with dielectric material to prevent the effects of variation of pressure and weather on the radiation of energy therefrom.

While there has been described what is at present considered an illustrative embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. An antenna for radar systems comprising a primary reflector having a symmetrical parabolic cylindrical concave reflecting surface terminated by a pair of edges substantially parallel to its focal line, a pair of plate members disposed with their planes substantially perpendicular to the focal line of said primary reflector and adjacent to the ends of said primary reflector, a rectangular Waveguide having one end adapted to be connected to a source of electromagnetic energy and a plane open end positioned substantially parallel to the plane determined by said pair of edges to illuminate the reflecting surface of said primary reflector with said energy, said radiating end being located outside the space bonded by the reflecting surface of said primary reflector and the plane determined by said pair of edges, and a secondary reflector having a generally parabolic cylindrical reflecting surface positioned to be illuminated with energy from said primary reflector and at a greater distance from said primary reflector than said radiating end of said waveguide, said secondary reflector being disposed with its focal line parallel to the planes of said plate members, one of the edges of said secondary reflector being substantially in the plane of one of said plate members, said one plate member extending the distance between said primary reflector and to said secondary reflector.

2. An antenna according to claim 1 in which said plate member is spaced capacitively from said one edge of said secondary reflector and said secondary reflector is tiltable about its focal line.

3. An arrangement for illuminating a directive reflector comprising a parabolic cylindrical reflecting surface having a focal line, conductive end plates perpendicular to said focal line and adjacent to said reflecting surface, the terminations of said reflecting surface and one of said end plates being contiguous with each other to form a radiating aperture positioned intermediate said'focal line and the vertex line of said reflecting surface, the other of said end plates being extended beyond said radiating aperture to a position adjacent to the directive reflector to be i1- luminated, and means positioned on said focal line to illuminate said reflecting surface with polarized electromagnetic energy, said last-mentioned means being oriented to provide electromagnetic energy polarized parallel to said end plates.

4. An arrangement for illuminating a main directive reflector comprising a first directive reflecting surface having a focal line, at least one conductive end plate perpendicular to the focal line and adjacent to said first reflecting surface, the intersection of the terminations of said first reflecting surface and said end plate forming an aperture for radiating energy onto the main directive reflector, said focal line being outside of the space bounded by said aperture and said reflecting surface, said end plate being extended beyond said radiating aperture to a position adjacent the main directive reflector, and means positioned on said focal line for illuminating said first reflecting surface with polarized electromagnetic energy, said last-mentioned means being oriented to provide electromagnetic energy polarized parallel to said end plate.

5. A directive antenna with means for distorting the radiation pattern from said antenna into a csc 0 pattern in one plane, where 0 is the angle measured from the axis of directivity of the undistorted radiation pattern, comprising a first parabolic cylindrical reflecting surface having a first focal line, a second parabolic cylindrical reflecting surface having a second focal 10 and radiating means positioned to illuminate said first reflecting surface.

WILLIAM SICI-IAK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 20 Number Name Date 1,898,197 Ludenia Feb. 21, 1933 2,118,419 Scharlau May 24, 1938 2,206,923 Southworth July 9, 1940 2,283,925 King May 26, 1942 2,407,068 Fiske et al Sept. 3, 1946 2,421,593 Bishop June 3, 1947 2,434,253 Beck Jan. 13, 1948 2,436,408 Tawney Feb. 24, 1948 FOREIGN PATENTS Number Country Date 636,809 Germany Oct. 1'7, 1936 495,977 Great Britain Nov. 23, 1938 

